Researchers at Nanyang Technological University (NTU) report in an open access paper in Scientific Reports that bulk antimony sulfide (Sb2S3) with a size of 10–20 μm used as a Li-ion electrode material exhibits a high capacity and stable cycling of 800 mAh g−1. Despite the large particle size, bulk antimony sulfide also showed excellent rate performance with a capacity of 580 mAh g−1 at a rate of 2000 mA g−1—the highest ever recorded for materials with 10–20 μm size.

The mechanical and chemical stabilities of the electrodes were ensured by an optimal electrode-electrolyte system design, with a polyimide-based binder together with fluoroethylene carbonate in the electrolyte, the authors said.

Nanomaterials as anodes for lithium-ion batteries (LIB) have gained widespread interest in the research community. The NTU team had earlier collaborated on antimony sulphide nanoparticle-coated graphene materials which yielded a high capacity of 730 mAh g−1 at 50 mA g−1, an excellent rate capability up to 6C and a good cycle performance. (Earlier post.) However, the authors note in their new paper, scaling up and processability are bottlenecks to further commercialization of these materials.

In this article, our focus is on alloy-based metal sulfide (MxS) as anode materials because they can undergo both conversion reaction between Li and MxS and alloying reaction of the remaining metal with Li, resulting in higher charge and discharge capacities. Capacity degradation observed in these materials is typically attributed to large volume expansion/contraction during charge and discharge, and also dissolution of polysulfide from the active material. However, very few studies attempted to isolate the mechanical and chemical influences on the stability of the electrode. In particular, most of the research work on metal sulfides focused only on the synthesis and application of nano-sizes, nanomorphologies and graphene-based composites to suppress the volume expansion.

Here we want to identify and overcome the challenges when bulk materials are used. In this study, we have shown that particle size is in fact not the most crucial factor on the stability of the active materials. We achieved excellent cycle performance and rate capability using bulk materials by ensuring the mechanical and chemical stabilities of the electrodes.

To understand the mechanical and chemical stabilities of the electrode, the effects of binder and electrolyte additive on the cycle performance of the metal sulfide electrodes were analysed systematically. Antimony sulfide (Sb2S3) is chosen as the model system for this work because it is commercially available and can also be synthesized easily. Sb2S3 also has a working potential of around 1 V vs. Li/Li+, far away from the potential of Li dendrite formation, making it a safer anode material for fast charging applications. Different factors affecting the electrochemical performance of the Sb2S3 electrode are identified and the issues are resolved by using a proper electrode-electrolyte system. These results can be equally applicable to other metal sulfides materials such as SnS and In2S3, etc.

—Yu et al.

They found that the polyimide binder accommodates the volume expansion during alloying process and fluoroethylene carbonate suppresses the increase in charge transfer resistance of the electrodes.

After determining that bulk Sb2S3 exhibited high capacity and good cycle stability with a suitable binder and electrolyte composition, they compared it with electrodes made from Sb2S3 wires with a width of about 1 μm. The were tested with FEC/DEC electrolyte, the same as the one with bulk Sb2S3.

Both Sb2S3 wires and bulk Sb2S3 electrodes showed similar charge-discharge peaks for alloying and conversion reactions of Sb2S3. Other than additional peaks at 1.9 and 2.35 V during charging and 1.7 V during discharging in the electrode with Sb2S3 wire, both electrodes showed similar peak currents, indicating similar charge-discharge capacities, the researchers found. Sb2S3 wire showed slightly higher capacity than bulk Sb2S3 at a rate of 250 mA g−1, but the difference was not significant, they determined.

With the polyimide binder providing mechanical stability to the electrodes and the FEC suppressing surface reaction, both electrodes gave good cycle performance. The electrodes were also tested up to 2000 mA g−1, yielding the same rate performances were observed for both Sb2S3 wire and bulk Sb2S3. This indicated that no additional benefits were observed by reducing the particle size, and the reduction of particle size is not necessary for obtaining high rate and high capacity in these materials, the team concluded.

Capacity and cycle performance of electrode materials are highly dependent on electrode configuration and electrolyte composition. Systematic investigations were carried out to analyse the factors affecting cycle performance. In the case of Sb2S3, volume change and surface reactions were identified as the major factors affecting the electrode stability. Polysulfide dissolution is not a main factor for short-term stability The use of polyimide binder allows higher utilization of the Sb2S3 active material and provides good mechanical stability in the electrodes. In fact, some polyimide binders are active with Li and can lead to additional capacity in the electrode. Surface degradation and impedance change is suppressed by the use of FEC in the electrolyte.

… After judicial choosing the binder and electrolyte, electrodes with bulk Sb2S3 gives a charge capacity of close to 800 mAh g−1 with stable cycle stability. Particle size effect is not found to be crucial once the electrode configuration and electrolyte is optimized. Nano-particles, which are difficult to handle and synthesize in large-scale, are not necessary to provide good electrochemical performances.